In industrial quality control performed on products with surfaces comprising low-scale structured (relief-like) surfaces with regard to verification for defects of fabrication reflected by a specific anomaly of the shaping of the surface, contactless real-time measuring methods may be employed. Particularly in characterizing vehicle tires with regard to side or height wobble it is essential to recognize bulges or constrictions, on the one hand, and inscriptions and/or markings applied to the tires, on the other hand, so that they do not interfere with characterizing the vehicle tire.
The particular difficulty in detecting side or height wobble is the fact that raised, relief-like graphic characters or markings have been applied, as a rule simultaneously, on the areas to be tested, and that the anomaly structure is situated at the same height interval as the writing or at a lower height interval than same, it being possible that the tire surface to be tested additionally has a torus-shaped curvature.
As a consequence, correct measuring of the anomaly of height is distorted or, in many cases, even prevented by the presence of the structures of writing.
The production of vehicle tires may give rise to defects of fabrication in the inner structure which significantly influence the mechanical properties and therefore the operating behavior. It is necessary to discard any such products. In vehicle tires, such production defects may occur on the side faces or running treads, and typically become apparent in the form of deviations from a radially symmetric surface. The extensions of such imperfections are generally higher than the constructive structures which are present on the surface at the same time, such as relieves of writing or marking.
So far, mainly capacitive measuring methods have been employed for this test assignment in the field of industry, which, however, can only provide an insufficient testing depth for the reasons laid down below. While the surface is moving, i.e. while the tire is rotating, a change in the distance between the measuring electrode and the tire surface is determined by means of a change in the capacity of a measuring sensor. The distance lies in its relatively coarse lateral spatial resolution due to its geometry, which implies that only a small number of tracks per width of the testing range may be suitable for testing. The measurement signal contains no sufficient information on the precise course of a deflection in height, so that any short-range structures (relief or bulges/constrictions) cannot be differentiated and blanked out for further calculation.
In addition, certain tire manufacturers employ tactile (contacting) measuring methods, which, however, place certain requirements upon the geometry of the objects to be measured. The limitation is that on the surface to be examined, an explicit track to be measured must be provided which must not comprise any constructive structures, and the minimum width of which must include the geometry of the measuring sensor and potential irregularities in the concentricity during measuring. Under these circumstances, the measuring method is capable of determining relatively reliable measured values. However, due to the geometric limitations and the current tendency, conflicting therewith, to produce very narrow tire side flanks (tires with a small cross-section), it does not provide a satisfactory measuring method for general use.
A known method for measuring surface contours is optical triangulation. It includes focusing a light beam (generally a laser beam) onto the surface to be measured, and optically imaging the diffusely reflected radiation onto a sensor with a plurality of picture elements (pixels). If the geometry between the camera and the light beam remains unchanged, the change in the spatial position of the light intersection point on the object to be measured along the beam may be determined from a shift of the point of light reflected on the sensor area. Such a measurement is initially performed point by point. If a whole region is to be tested, the object under test is moved along beneath the triangulator measuring arrangement, and the measured values are recorded in a fast sequence, so that a narrow, annular track on the tire side face is detected.
With the increasing availability of high-performance laser light sources and optical sensors, bulge testing systems on the basis of laser triangulation have recently been offered. These systems are capable of capturing a specific track to be measured on the tire surface at a high spatial resolution, and to evaluate same with regard to potential shape anomalies. However, since these methods are not able to reliably recognize constructive structures of writing, the height of the shape anomalies is distorted by the height of the regular structures of writing. This is added to by the problem that the maximum amplitude of any occurring shape anomalies is not necessarily situated on the selected track to be measured.
A known derivative of the described triangulation which is also known includes sampling the surface by means of fan beam and area sensor (light section method). The height-related information may be determined along the measuring line on the surface by means of the light stroke projected onto the sensor (line of intersection between the fan beam and the object surface). By moving the object, the height-related information along this line is recorded row by row and subsequently combined to form a complete 2D height image comprising the respective height-related information in each image point. With sufficient resolution of the measurement in terms of space and time, the data set thus produced contains the height-related information derived from an entire surface region, including the structures of defects and writing and/or marking. However, since structures of writing and defects are situated in the same range of height, and since these structures are situated on a surface which is strongly curved in relation to these structures, it is not possible—even by means of the height images thus acquired—to achieve a secure differentiation between the respective relief structures in a simple manner without special data processing methods.
WO 00/25088 discloses methods and apparatus for determining points of unevenness in a domed surface, such as a tire side wall, using band-pass filtering. In particular, a three-dimensional surface representation of the tire side wall is created. Hereupon, the doming is extracted from the three-dimensional representation of the surface, and the edges of the structuring, such as of a writing or of markings, are smoothened to obtain a domeless representation of the domed surface. The surface now contains the potentially present unevenness, such as a constriction or a bulge on the tire side wall, but the edges of the inscription are now smoothened. Subsequently, the domeless representation is compared to a threshold so as to determine two dimensional regions of the domeless representation which have a predetermined relation to the threshold value. Eventually, the areas of the determined regions are evaluated, wherein a region is detected to be an unevenness if its surface area is larger than a predetermined surface area. The relief-like structures of writing on the tires no longer play a role in evaluating the areas, since the edges of these inscriptions have been smoothened, and thus the heights of the inscriptions have been reduced such that they no longer exceed the threshold value at all, or such that they now have only a small area exceeding the threshold value. In the evaluation of the size of the areas exceeding the threshold value, these small areas may readily be distinguished from those areas which are due to bulges or constrictions. For extracting the surface and for smoothing the edges, a band pass filter is preferably used which has an upper and a lower cut-off frequency, the lower cut-off frequency being set such that the doming is suppressed, and wherein the upper cut-off frequency is set such that the edges are smoothened, whereas the points of unevenness in the form of bulges or constrictions substantially are not adversely affected.
Although, this method already brings substantial advantages compared to one-dimensionally working measurement methods, also here certain restrictions regarding testing capability exist, in particular regarding the maximum admissible height extension of the constructive structures, i.e. the structurings restricted by edges on the device under test. For a secure defect detection it needs to be guaranteed that the writing structures have at most the same height as the defect structures to be detected. Not rarely is it the case that a writing, i.e. a target structuring limited by edges is substantially higher than the defect to be determined in the form of bulges or constrictions. In the identification of substantially smaller defect structures than writing structures and also for the case that the writing structures are, however, present in the same height region but clearly expanded laterally, no secure difference between edgeless unevennesses and structurings limited by edges is possible any more. As the structurings are to be blanked out by low-pass filters, a secure blanking out is only guaranteed, when the structuring is not too high and not too large. If, however, a structuring which is too high or too large, respectively, is smoothed it becomes an artificially generated substantially edge-free unevenness after the low-pass filtering which may wrongly be seen as a bulge. If the structurings have a negative height extension, as it may for example occur by a stamp which is pressed into the tire surface, then this stamp may wrongly be seen as a constriction as it does not have any sharp edges anymore after low-pass filtering, has, however, not been suppressed so strong in order for its depth to be smaller than the critical depth predetermined for constrictions.